1. Field of the Invention
The invention relates to latent inks and coatings intended to be substantially undetectable except under specific activation conditions that may be kept confidential or if known are not easily arranged. When used to apply foreground text or symbols to a background, such inks and coatings sometimes are termed “invisible ink” indicia. The ink or coating alternatively can be used as a distinct detectable background feature.
The invention provides a latent ink formulation for inkjet printing and coating applications, comprising insoluble phosphor pigment particles that emit light by phosphorescence resulting from excitation, wherein emission is at a different wavelength than excitation. In the preferred formulation, a pigment is used that has two distinct phosphorescent response wavelengths in response to illumination at two different excitation wavelengths. Preferably, the response wavelengths are within the visible spectrum and the excitation wavelengths are not.
The inkjet formulation of the invention is readily produced and provides a robust phosphorescent response to excitation while being substantially indistinguishable from an uncoated substrate (i.e., invisible) in the absence of excitation
2. Prior Art
U.S. Pat. No. 5,592,561—Moore discloses an authenticating, tracking/anti-diversion, and anti-counterfeiting system that can track various goods. The system includes a control computer, a host computer, a marking system, and a field reader system, which can be physically linked via data transmission links. An identifiable and unique mark is placed on each good, or on materials out of which the goods are to be made, which enables subsequent inspection. The marks or patterns include areas where a marking agent is applied in an encrypted pattern and areas where it is not applied. The pattern can be scanned or captured by a reader and deciphered into encoded data. The entry can then either be compared directly to a set of authentic entries on a database or decoded and compared to a set of data on the centrally located host database. The marking system provides control over imprinting, allowing a limited number of authorized codes to be printed before re-authorization is required. Monitoring of the marked goods is facilitated by including a unique encrypted pattern having, for example, a unique owner identifier, a unique manufacturer identifier, a unique plant identifier, a unique destination identifier, and time and date information.
U.S. Pat. No. 5,574,790—Liang et al. discloses a multiple-reader system for authentication of articles based on multiple sensed fluorescent discriminating variables, such as wavelengths, amplitudes, and time delays relative to a modulated illuminating light. The fluorescent indicia incorporates spatial distributions such as bar codes as discriminating features, to define a user-determined and programmable encryption of the articles' authentic identity.
U.S. Pat. No. 5,289,547—Ligas et al. discloses a method for authenticating articles including incorporating into a carrier composition a mixture of at least two photochromic compounds that have different absorption maxima in the activated state and other different properties to form the authenticating display data on the article, subjecting the display data to various steps of the authenticating method, activating all photochromic compounds, preferential bleaching of less than all of the photochromic compounds, and/or bleaching of all the photochromic compounds, and subsequent examination of the display data following the various activation and bleaching steps by verifying means to enable authentication.
U.S. Pat. No. 5,974,150—Kaish et al. discloses an anti-counterfeiting system wherein an authentication certificate affixed to a product is impregnated with dichroic fibers containing a fluorescent dye. Dichroic polymer fibers may also form part of the product to be authenticated. In order to determine if the imprinted code corresponds to the certificate itself, the fiber pattern, which is completely random, is illuminated by a light and read by a scanner. The resulting pattern is then compared to the encoded pattern to determine authenticity.
U.S. Pat. No. 5,212,558—Obata et al. discloses an encoding system wherein a confidential image is recorded on a substrate using invisible ink. The invisible ink emits visible light having a wavelength of about 360-380 ηm when irradiated with light having a wavelength of 250 ηm. Unfortunately, such a system is easily compromised by viewing the invisible ink with a black light, which is readily available to the public.
Similarly, U.S. Pat. No. 5,939,468—Siddiqui discloses jet ink compositions suitable for producing marks on objects that are invisible to the unaided eye and are visible only when excited by exciting radiation in the preferred wavelength region of from about 275 ηm to about 400 ηm.
U.S. Pat. No. 5,093,147—Andrus et al. discloses a method for providing intelligible marks that are virtually invisible to the unaided eye on the surface of an article. The invention is based on a jet ink containing an organic laser dye that is poorly absorptive in the visible range of about 400 to 700 ηm, is absorptive of radiation in the near infrared range of at least 750 ηm, and fluoresces in response to radiation excitation in the infrared range at a wavelength longer than that of the exciting radiation. Thus, the marks remain invisible to the naked eye after excitation.
Additional related disclosures of fluorescing pigments for normally invisible or latent markings are found in EPO 595,583—Canon KK, PCT/US98/04672 (WO 98/40223)—Polaroid Corp., PCT/US97/20342—Eastman Chem. Co., DE 198 03 997—Kaule et al., and FR 2,739,324—Schiffmann et al. Examples of phosphors for authentication purposes having different excitation and emissions wavelengths are disclosed, for example, in U.S. Pat. No. 4,387,112—Blach.
U.S. Pat. No. 5,935,308—Siddiqui et al. and U.S. Pat. No. 5,395,432—Nelson et al. disclose a latent image printing process involving activation prior to detection of the latent image. A latent image is applied to a substrate using an ink composition containing zinc chloride or zinc bromide. These references apply the normally-invisible (latent) image to a substrate using an ink jet printing technique. The latent image is activated by application of a fluid activator that reacts with the applied image and renders the printing permanently visible thereafter. Activation techniques of this type may be useful in some instances but they can be checked for security purposes only once, namely activated and viewed, whereupon the image is no longer latent and the security aspect is lost or very much reduced. The marking is not as useful as a mark that remains latent between detection steps or returns to its latent state after activation. The permanently visible activated marking in the Siddiqui and Nelston technique may tend to devalue some types of marked substrate due to the permanent activated mark. As another aspect of these prior art references, which could be viewed either as a security drawback or a saving grace, the latent image can be removed using a solvent or diluted to the point of erasing the image, because zinc chloride and zinc bromide have high solubility in many common solvents.
All the foregoing references are hereby incorporated for their teachings of phosphors and coatings for authentication uses. According to the present invention, certain insoluble phosphors are employed in an optimal ink formulation for inkjet applications.